Age-related changes in lens properties are due partially to the very gradual growth of the lens and partially to the cumulative photochemical consequences of ambient exposure to the near-ultraviolet and blue wavelengths absorbed by the lens. It is not uncommon to find that lens aging processes have progressed to the extent where vision is significantly degraded, while other ocular tissues have yet to exhibit noticeable degenerative effects. Because lens tissue never sheds any of its cells, the photoproducts induced by ambient light exposures accumulate within the lens. As a result, the near-uv absorption observed in the young lens begins to impinge upon the short, visible spectrum so that there is a gradual attenuation of blue light in the aging lens, and the initially clear lens takes on an increasingly yellowish and ultimately brunescent coloration.
Concomitant with the age-related changes in lens absorption and scattering is a corresponding evolution of the lens autofluorescence. The fluorescence from lens photoproducts, being close molecular derivatives of the near-uv absorbing, blue-fluorescing chromophore found in the young lens, becomes stronger and encompasses longer visible wavelengths in the aging lens. In the aged lens (&gt;80 yr), a bright ambient environment (normal sunlight) can induce a fluorescence glare of sufficient intensity to impair visual function.
Although the etiology of cataract formation is still under investigation, there is a general consensus that photochemically induced changes in lens molecular composition ultimately result in cataract expression. This engenders further changes in the visible light absorption, scattering and fluorescent properties of the aging lens. Numerous investigations have examined the age-related and cataract-associated optical changes as means for monitoring signs of premature aging or early indication of cataract. In addition, diseases including diabetes and use of photosensitizing drugs may precipitate relatively rapid cataract formation, and characteristic changes in the optical properties of the lens which presage this consequence have been reported.
Various experimental approaches to characterizing the optical qualities of the lens have been reported. Some are noninvasive and possibly suitable for in vivo measurements in the alert human patient. Others are too time-consuming and tedious for use with alert subjects. Yet others require the exposure of the subject to undesirable levels of near-UV and visible radiation. Often, existing approaches, while measuring specific aspects of the lens spectral properties, do not give a sufficiently complete optical profile to be of value in assessing the overall impact of aging, in tracking progress towards eventual cataract formation, or in discerning the effects on the lens of other diseases or of various photosensitizing drugs.
Accordingly, an object of the present invention is to provide a rapid, noninvasive and safe method for collecting extensive quantitative optical data from human lenses in order to discern departures from normalcy.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.